1. Field of the Invention
The present invention relates to hydrostatic transmissions intended primarily for use in the lawn and garden industry on riding lawn mowers, lawn and garden tractors and the like, but may also be applied to larger implements and vehicles. More particularly, the present invention relates to a neutral brake system for a hydrostatic transmission to automatically apply a brake when the transmission is shifted to neutral.
2. Description of the Related Art
Hydrostatic transmissions transmit rotary mechanical motion from an internal combustion engine to fluid motion, typically oil or hydraulic fluid, and then back to rotary mechanical motion to rotate a pair of drive axles in order to drive a vehicle. Hydrostatic transmissions generally include a pump, such as a variable displacement pump, driven by the engine. The pump drives a motor, such as a fixed displacement motor, through a hydraulic circuit between the pump and the motor. The output speed of the hydrostatic transmission is typically controlled by varying the displacement of the pump using a movable track ring or swash plate. An example of such a hydrostatic transmission is described in U.S. Pat. No. 5,182,966, assigned to the assignee of the present application, and expressly incorporated herein by reference.
When the hydrostatic transmission is shifted to neutral, the displacement of the pump is zero, there is substantially no hydraulic pressure in the hydraulic circuit, and the motor is idle. When the hydrostatic transmission is shifted out of neutral, the displacement of the pump provides a charge of hydraulic pressure to the hydraulic circuit to drive the motor. The motor in turn drives a pair of drive axles through a reduction gear train and a differential mechanism.
The motor, reduction gear train, differential mechanism, and drive axles are disengaged from the engine when the hydrostatic transmission is shifted to neutral, and, if the vehicle is parked on an inclined surface it will be free to roll downhill. Typically, a user sets a manual parking brake or keeps pressure applied to a foot brake when the transmission is shifted to neutral to prevent the vehicle from rolling downhill.
Problematically, vehicles having hydrostatic transmissions with a manual parking brake and/or a foot brake are dependent upon the user to actuate the brakes to prevent the vehicle from moving when the hydrostatic transmission is shifted to neutral.
Other known hydrostatic transmissions include a hydraulic circuit between a motor and a pump, with the displacement of the pump controlled by a control system such as a hydraulic servo, which may be part of a second hydraulic circuit. Typically, these hydrostatic transmissions also include complex arrangements of servos, multiple valves, and control levers. For example, a user may manually apply or release a hydraulic friction brake after the user first manually actuates a servo and valve control mechanism which destrokes the variable displacement pump to maintain the transmission in a minimum drive condition. Alternatively, a control valve located in a second hydraulic circuit may be responsive to a pilot hydraulic pressure in the main line hydraulic circuit. The control valve in turn controls the hydraulic fluid supply to a hydraulic servo, also located in the second hydraulic circuit, which servo controls the pump, in order to prevent the main line pressure from substantially exceeding a predetermined level.
The foregoing hydrostatic transmissions are disadvantageous because they are complex and require several components such as, for example, a second hydraulic fluid source, servo devices, and multiple control and shuttle valves. Maintenance of such transmissions can also be time-consuming and expensive. Further, such hydrostatic transmissions are not easily retrofittable into existing hydrostatic transmissions.
What is needed is a neutral brake system for a hydrostatic transmission which is automatically engaged to stop the vehicle from moving when the hydrostatic transmission is shifted to neutral.
A further need is for a neutral brake system which does not have a complex design, such that the number and complexity of the components which comprise the brake system is reduced, to simplify and reduce the cost of maintenance.
Still another need is for a neutral brake system which may be easily retrofitted into existing hydrostatic transmissions.
The present invention is a neutral brake system for use with a hydrostatic transmission to automatically apply a brake when the hydrostatic transmission is shifted to neutral, including a hydraulically-released, spring-activated brake mechanism, the brake mechanism in fluid communication with, and controlled by, the main hydraulic passages which form a hydraulic circuit between the pump and the motor of the hydrostatic transmission.
A brake release cylinder having a piston therein is communicated directly to the closed hydraulic circuit between the pump and the motor by a pair of brake passages via a shuttle valve. When the hydrostatic transmission is shifted to neutral, the hydraulic pressure in the hydraulic circuit is substantially zero, and a brake spring biases a brake lever into contact with a friction pad. The friction pad engages a brake disk splined to the output shaft of the motor to stop rotation of the output shaft, and to thereby stop the vehicle from moving. When the hydrostatic transmission is shifted out of neutral, the pump produces a charge of hydraulic pressure within the hydraulic circuit, and a piston within the brake release cylinder overrides the bias force of the brake spring, and urges the brake lever out of contact with the friction pad to release the brake disk such that the output shaft may freely rotate.
In one form thereof, a brake system in combination with a hydrostatic transmission is provided, the hydrostatic transmission having a hydraulic circuit between a pump and a motor, the brake system including a hydraulically-released, spring-activated brake mechanism in direct fluid communication with the hydraulic circuit, the hydraulic circuit controlling the hydraulic brake mechanism.
In another form thereof, a hydrostatic transmission is provided, including a pump, a motor in fluid communication with the pump, the pump and motor forming a hydraulic circuit, and a hydraulically-released, spring-activated brake mechanism in direct fluid communication with the hydraulic circuit, the hydraulic circuit controlling the hydraulic brake mechanism.
The advantage of this arrangement is that the brake mechanism is automatically engaged when the hydrostatic transmission is shifted to neutral, obviating the need for a user to manually set a parking brake or to keep pressure on a foot brake to stop the vehicle from moving when the hydrostatic transmission is shifted to neutral.
A further advantage of this arrangement is that, because the brake mechanism is in direct fluid communication with the hydraulic circuit so that the number and complexity of the brake mechanism components is substantially reduced, which in turn reduces assembly and maintenance costs.
Still another advantage of this arrangement is that the brake mechanism may be easily retrofitted into existing hydrostatic transmissions.